Fewer school staff may be “willing or able” to work following the Prime Minister’s move to lock down Britain in the absence of clear advice on how to stay safe in schools, head teachers have said.

Many people will be “rightly” concerned about their families and anxious about the risks of going to work after hearing Boris Johnson’s announcement, according to the National Association of Head Teachers (NAHT).

Schools in England officially closed on Friday but have remained open for vulnerable youngsters and the children of key workers, such as NHS staff, police and delivery drivers.

In a message to members, NAHT general secretary Paul Whiteman said: “We know that the vast majority of schools opened yesterday offering reduced provision. Our survey showed that most schools operated with fewer than one in four teaching and support staff present.

“Having heard the Prime Minister speak, many colleagues will be rightly concerned for loved ones and understandably anxious about the personal risk of simply going to work. It would not be surprising to find, in the absence of clear Government advice on keeping safe, that fewer colleagues are willing or able to work today than yesterday.”

Mr Whiteman added that in recent days, school leaders and their teams have shown “almost infinite reserves of resourcefulness” as he called for a “step change” in advice given to schools.

“Our work with the Department for Education is productive but we need a step change in the detail of advice given to schools now,” he said.

“Our focus will be to reiterate the need for schools to be given clear guidance on how to keep pupils and staff safe.

“It is absolutely vital that we have sight of the expert medical evidence on safe levels of attendance and density, and are provided with practical advice on the protective steps we should be taking in schools.

“From masks or other PPE, to distancing children from each other, to sufficient supplies of soap and hand sanitiser, schools urgently need answers to their questions about effective safety measures.”

There also needs to be more guidance for special schools and alternative provision, as these are taking in higher percentages of pupils, he said.

“School staff know that they are taking a risk by reporting to work, against advice given to the rest of the population to stay at home,” Mr Whiteman added.

“It is only right that this is a choice informed by hard evidence regarding the degree of risk involved.”

A Government spokesman said: “We are closely monitoring the situation and will continue to work with local authorities to ensure schools and colleges get the help and support they need over the challenging weeks and months ahead.

“We are urging people to do everything they can to stay at home, protect the NHS and save lives.”

It is understood that more guidance for school staff on health and safety, such as social-distancing measures in schools, is currently being drawn up.

Education Secretary Gavin Williamson has thanked school staff for their efforts.

In a tweet, he said: “Thank you to all the teachers and staff working hard to support those who need it most.

“Every one of us is enlisted in the fight against this virus.”

Early indications suggest that the numbers of pupils arriving at schools is lower than first expected by school leaders, as parents heed advice to keep their children at home unless they have no other option.

An NAHT survey of 3,350 members has found that 94% of schools are providing emergency cover for pupils, and 84% of them have less than a fifth (20%) of their usual children attending.

A separate snapshot survey of 670 schools, mainly secondary, showed that in most cases, fewer than 10% of pupils were in school on Monday.

ASCL, which conducted the poll, said that in 86% of schools, the percentage of pupils attending lessons was between 1% and 5%.

Most of the schools were in England (94%), with the rest in Wales and Northern Ireland.

Downing Street said attendance at schools in England was around 10% of normal levels on Monday.

The Prime Minister’s official spokesman said: “In terms of yesterday, I believe that the number of children attending school was around the 10% mark.”